Airtel — mobile: postpaid to prepaid plan

Address:

Bhopal, 462016

I visited airtel store on 25/11/2019 located at bittan market bhopal, where i asked to change my plan from postpaid to prepaid plan
The executive asked me to pay due amount of rs 270 to change to prepaid.
I deposited the due amount after which the executive told that within an hour my plan will change to prepaid from postpaid.
On not updation of plan till today (26/11/2019), i again visited above mentioned store, where he told that my due amount is rs 999.
After this i called airtel customer care - 121, mr vishal and airtel executice attended the call, in which he told that i have to wait for 1 month to complete billing cycle, and on completion of biiling cycle i will have to pay the bill after a month.
My greviance is that, i had to wait for the end of billing cycle then why the airtel store executive made me pay rs 270 on 25/11/2019. He should have told me to wait untill end of billing cycle.